There has been a demand for a thermally conductive material capable of transferring heat from a heat source to a cooling source or a thermally conductive material capable of being used in a bent state during transfer of heat from a heat source to a cooling source. This demand is intended to deal with complication of internal structures of electronic devices. As such a thermally conductive material, conventionally, a metal material (e.g., copper or aluminum) such as metal foil has been used.
However, in recent years, the amount of heat which electronic devices generate has been increasing. Accordingly, a required thickness of a metal material has increased. This has been leading to insufficient flexibility of the metal material. Meanwhile, an increase in weight of the metal material has been also a problem. As a possible material which may solve the above problems, there is a graphite composite material in which graphite sheets each having a high thermal conductivity and lightweight properties are stacked in a layer stack. However, there has been a problem that bendability of a graphite composite material decreases in a case where graphite sheets are stacked via an adhesive or the like so as to form a layer stack as in Patent Literature 1. On the other hand, in a case where graphite sheets are stacked via no bonding layer, a gap is produced between graphite sheets, so that a contact thermal resistance disadvantageously increases. This has led to a problem that heat conductivity largely deteriorates at a part where heat from a heat source is received.
Further, in recent years, the amount of heat which electronic devices generate tends to increase as performance of the electronic devices improves. This has caused a problem that a required amount of a thermally conductive material to be used increases. Further, in a case where an electronic device is provided in a device such as a mobile medium or a driving section, vibrations generated by the device is transmitted directly to the electronic device. There has been a concern that such vibrations may cause a serious adverse effect to the electronic device. Further, there has been also a concern that the vibrations may cause metal fatigue of a thermally conductive material and may consequently lead to a decrease in heat conductivity of the thermally conductive material. In this way, reliability against vibrations has been low in a case where the metal material is used as a thermally conductive material. The phrase “reliability against vibrations is low” means that vibrations largely damage an electronic device and consequently, heat conductivity of the thermally conductive material is likely to deteriorate. Meanwhile, the phrase “reliability against vibrations is high” or “reliability against vibrations is sufficient” means that damage to an electronic device is small and accordingly, heat conductivity of the thermally conductive material is unlikely to deteriorate.
Patent Literature 1 discloses a thermally conductive body formed by laminating graphite films via a polymer layer. However, Patent Literature 1 does not disclose any thermal transport body capable of efficiently transporting heat. Further, Patent Literature 1 discloses a structure in which strip-shaped graphite sheets are bundled. However, in this structure, a part at which the graphite sheets are bundled is covered by another material. Accordingly, it is difficult to use the structure as a thermal transport structure which can be freely bent or which can prevent reliability deterioration caused by vibrations.